The present invention relates to a magnetic recording medium using a ferromagnetic metal thin film, and more particularly to a magnetic recording medium having excellent electromagnetic transducing properties, and a large capacity magnetic recording and reproducing apparatus.
For improving the recording density, increasing the output and reducing the noise of magnetic recording media, it is essential to micronize magnetic particles in the case of a coated medium and crystal grains in the case of a thin film medium. Regarding a medium using metal particles that has heretofore been studied, for example, micronization has progressed and high-performance tapes such as Hi-8 (8-mm high-density magnetic tapes) using extra-fine particles having a cylinder major axis length of approximately 200 nm and a cylinder diameter of approximately 30 nm are now put to practical use. Incidentally, a plurality of particles are subjected to magnetic reversal in a group and signals are recorded when magnetic particles have been formed into a cluster agglomerate or when the interaction between crystal grains is strong even though the magnetic particles or crystal grains of a magnetic medium are extremely fine. When the plurality of particles are subjected to magnetic reversal and when the magnetic reversal unit becomes larger, noise increases at the time of reproducing data. In consequence, the density improvement is greatly hampered.
The size of the magnetic reversal unit is relevant to magnetic viscosity. In other words, it is considered that the greater the fluctuation field of magnetic viscosity becomes, the smaller the magnetic reversal unit is. A description has been given of a meaning of the fluctuation field of magnetic viscosity in the Journal of Physics F: Metal Physics, Vol. 14, pp L155 to L159 (1984). Further, a detailed description has also been given of the measurement conditions in the Journal of Magnetism and Magnetic Materials, Vol. 127, pp 233 to 240 (1993). The principle of measuring the fluctuation field of magnetic viscosity will subsequently be described.
When a new magnetic field is applied to a magnetic material, the magnetization I(t) often varies in relation to the logarithm lnt of the field applied time: EQU I(t)=const.+S.multidot.lnt. (1)
In this case, I(t) represents a magnetic moment per unit volume; and t, elapsed time after the new magnetic field is applied. The viscosity coefficients has a positive value when the magnetic field is shifted in the positive direction and has a negative value when it is shifted in the negative direction. Moreover, it is known that S can be expressed by the product of the irreversible susceptibility .chi..sub.irr and the fluctuation field H.sub.f. In other words, there is established. EQU S=.chi..sub.irr.multidot.H.sub.f (2)
Therefore, the fluctuation field is determined if S and .chi..sub.irr are found experimentally. The fluctuation field is a quantity representing the degree of the influence of thermal fluctuation, and a greater fluctuation field signifies that it is easily affected by thermal fluctuation and that the magnetic reversal unit is small in size.
The fluctuation field where the field strength is equal to coercivity or remanence coercivity can also be found from the dependence on the field applied time of the coercivity H.sub.c, or remanence coercivity H.sub.r, The coercivity or remanence coercivity, together with field applied time t, often lowers in relation to EQU H.sub.c (or H.sub.r)=-A.multidot.lnt+const. (3)
as the application time elapses. All the specimens mentioned in the present specification, satisfied the equation (3). When the coercivity or remanence coercivity varies with the field applied time t according to Eq. (3), it is known that A takes substantially the same value as that of the fluctuation field H.sub.f where the field strength is equal to the coercivity or remanence coercivity. This procedure is not only simple but also excellent in reproducibility. Hence, the value A is taken as the fluctuation field of magnetic viscosity according to the present invention.
By measurement at room temperature, the fluctuation field thus found has the nature of becoming large in proportion to the absolute temperature at the time of measurement. When a fluctuation field is measured at room temperatures ranging from 10.degree. C. to 30.degree. C. excluding 25.degree. C. (the absolute temperature: T) according to the present invention, the fluctuation field thus measured is multiplied by (298/T) to take the production as a fluctuation field H.sub.f at 25.degree. C.
In accordance with the conventional method, a Cr under-layer was first formed on a mirror-polished disk made of Ni--P electroless-plated Al--Mg alloy, and then a CoCrTa magnetic layer together with a protective carbon film was formed thereon to fabricate a magnetic disk. The Cr under-layer, the magnetic layer and the protective layer were formed by Ar-gas sputtering. In this case, the substrate temperature and the Ar pressure were 300.degree. C. and 2.0 milliTorr, respectively. Further, the Cr under-layer, the magnetic layer and the protective layer were 50 nm, 25 nm and 10 nm thick, respectively. The composition of the CoCrTa magnetic layer is Co: 80%, Cr: 16%; Ta: 4%, expressed by atomic %. This composition will be expressed as CoCr.sub.16 Ta.sub.4. The coercivity H.sub.c and the remanence coercivity H.sub.r were 1645 and 1655 oersteds, respectively. Further, the fluctuation fields of magnetic viscosity at 25.degree. C. at the field strength equal to the coercivity and at the field strength equal to the remanence coercivity were 13.5 and 13.2 oersteds, respectively. Thus the fluctuation fields of magnetic viscosity at 25.degree. C. at the field strength equal to the coercivity and at the field strength equal to the remanence coercivity exhibit substantially the same value: hereinafter these are called simply the fluctuation field in this specification. Incidentally, the measuring time of the fluctuation field ranged from 0 to 30 minutes.
A parmalloy head having a gap length of 0.4 .mu.m and a coil of 24 turns was used to record magnetic data on the medium, and a magneto-resistive parmalloy head was used to reproduce the data in order to examine the electromagnetic transducing properties. The flying height at the time of recording and reproducing data was 80 nm then. As a result of measurement, noise at a longitudinal bit density of 150 kFCI (kilo Flux Change per Inch) was 22 .mu.Vrms.
Although a magnetic disk unit having a recording density of 300 megabits/square inch could be fabricated by using this medium, a magnetic disk unit having a recording density of 1-gigabit/square inch could not be fabricated.
An object of the present invention is to provide a magnetic recording medium and a magnetic recording and reproducing apparatus suitable for reducing noise at the time of reproducing data and for high-density recording.